Hypoxia induces mitochondrial DNA damage and stimulates expression of a DNA repair enzyme, the Escherichia coli MutY DNA glycosylase homolog (MYH), in vivo, in the rat brain

Hypoxia-associated, acutely reduced blood oxygenation can compromise energy metabolism, alter oxidant/antioxidant balance and damage cellular components, including DNA. We show in vivo, in the rat brain that respiratory hypoxia leads to formation of the oxidative DNA lesion, 8-hydroxy-2'-deoxyguanosine (oh8dG), a biomarker for oxidative DNA damage and to increased expression of a DNA repair enzyme involved in protection of the genome from the mutagenic consequences of oh8dG. The enzyme is a homolog of the Escherichia coli MutY DNA glycosylase (MYH), which excises adenine residues misincorporated opposite the oxidized base, oh8dG. We have cloned a full-length rat MYH (rMYH) cDNA, which encodes 516 amino acids, and by in situ hybridization analysis obtained expression patterns of rMYH mRNA in hippocampal, cortical and cerebellar regions. Ensuing hypoxia, mitochondrial DNA damage was induced and rMYH expression strongly elevated. This is the first evidence for a regulated expression of a DNA repair enzyme in the context of respiratory hypoxia. Our findings support the premise that oxidative DNA damage is repaired in neurons and the possibility that the hypoxia-induced expression of a DNA repair enzyme in the brain represents an adaptive mechanism for protection of neuronal DNA from injurious consequences of disrupted energy metabolism and oxidant/antioxidant homeostasis.     

Ghrelin--not just another stomach hormone

Growth hormone (GH) secretagogues (GHSs) are non-natural, synthetic substances that stimulate GH secretion via a G-protein-coupled receptor called the GHS-receptor (GHS-R). The natural ligand for the GHS-R has been identified recently; it is called ghrelin. Ghrelin and its receptor show a widespread distribution in the body; the greatest expression of ghrelin is in stomach endocrine cells. Administration of exogenous ghrelin has been shown to stimulate pituitary GH secretion, appetite, body growth and fat deposition. Ghrelin was probably designed to be a major anabolic hormone. Ghrelin also exerts several other activities in the stomach. The findings that ghrelin is produced in mucosal endocrine cells of the stomach and intestine, and that ghrelin is measurable in the general circulation indicate its hormonal nature. A maximal expression of ghrelin in the stomach suggests that there is a gastrointestinal hypothalamic-pituitary axis that influences GH secretion, body growth and appetite that is responsive to nutritional and caloric intakes.     

Failure to produce mitochondrial DNA results in embryonic lethality in Rnaseh1 null mice

Although ribonucleases H (RNases H) have long been implicated in DNA metabolism, they are not required for viability in prokaryotes or unicellular eukaryotes. We generated Rnaseh1(-/-) mice to investigate the role of RNase H1 in mammals and observed developmental arrest at E8.5 in null embryos. A fraction of the mainly nuclear RNase H1 was targeted to mitochondria, and its absence in embryos resulted in a significant decrease in mitochondrial DNA content, leading to apoptotic cell death. This report links RNase H1 to generation of mitochondrial DNA, providing direct support for the strand-coupled mechanism of mitochondrial DNA replication. These findings also have important implications for therapy of mitochondrial dysfunctions and drug development for the structurally related RNase H of HIV.     

Heregulin is sufficient for the promotion of tumorigenicity and metastasis of breast cancer cells in vivo

Resistance of breast carcinomas to hormonal therapy is a clinical obstacle for the treatment of breast cancer. The molecular mechanisms and the factors involved in the progression of tumors from an estrogen (E2)-dependent to an E2-independent phenotype are not entirely understood. Heregulin (HRG) is a pleiotropic growth factor that binds to the erbB family of receptors, which are correlated with breast cancer progression and an aggressive phenotype in the breast carcinomas overexpressing the receptors. Previous studies in transgenic mice have shown that HRG is sufficient to induce mammary gland transformation and proliferation in the presence of hormonal stimulation. However, these studies did not address the important issue of the E2 independence that is part of the progression of breast cancer. In this study, we investigated the role of HRG in E2 independence. We were able to determine that HRG up-regulation was sufficient for the development of mammary tumors in the absence of E2 stimulation, a situation that mimics the progression of the human disease. We demonstrated that in ovariectomized nude mice, HRG induced E2 independence and antiestrogen resistance and promoted metastasis and preneoplastic transformation of the adjacent mouse mammary tissue. We show that one of the mechanisms by which HRG achieves the aggressive phenotype may be mediated via an increase in activated mitogen-activated protein kinase, an increase in a matrix-degrading enzyme, MMP-9, and the overexpression of vascular endothelial growth factors. The up-regulation of these genes occurred in the absence of any additional stimulation, in an autocrine manner. Our data provide new insights into the mechanisms of breast cancer progression in vivo, and reinforce the important role that HRG plays in this process.     

Oral cancer,cigarette smoke and mitochondrial 18 kDa translocator protein (TSPO) — In vitro,in vivo,salivary analysis

Oral cancer features high rates of mortality and morbidity, and is in dire need for new approaches. In the present study we analyzed 18 kDa translocator protein (TSPO) expression in oral (tongue) cancer tumors by immunohistochemistry. We also assayed TSPO binding in human tongue cancer cell lines and in the cellular fraction of saliva from tongue cancer patients, heavy cigarette smokers, and non-smoking healthy people as controls. Concurrently, TSPO protein levels, cell viability, mitochondrial membrane potential (Δψ_m), and general protein levels were analyzed. TSPO expression could be significantly enhanced in oral cancer tumors, compared to unaffected adjacent tissue. We also found that five-year survival probability dropped from 65% in patients with TSPO negative tumors to 7% in patients with highly expressed TSPO (p < 0.001). TSPO binding capacity was also pronounced in the human oral cancer cell lines SCC-25 and SCC-15 (3133 ± 643 fmol/mg protein and 6956 ± 549 fmol/mg protein, respectively). Binding decreased by 56% and 72%, in the SCC-25 and SCC-15 cell lines, respectively (p < 0.05) following CS exposure in cell culture. In the cellular fraction of saliva of heavy smokers TSPO binding was lower than in non-smokers (by 53%, p < 0.05). Also the cellular fraction of saliva exposed to CS in vitro showed decreased TSPO binding compared to unexposed saliva (by 30%, p < 0.001). Interestingly, oral cancer patients also displayed significantly lower TSPO binding in the cellular fraction of saliva compared to healthy controls (by 40%, p < 0.01). Our results suggest that low TSPO binding found in the cellular fraction of saliva may depend on genetic background as well as result from exposure to CS. We suggest that this may be related to a predisposition for occurrence of oral cancer.     

Electrical stimulation influences satellite cell proliferation and apoptosis in unloading-induced muscle atrophy in mice

Muscle atrophy caused by disuse is accompanied by adverse physiological and functional consequences. Satellite cells are the primary source of skeletal muscle regeneration. Satellite cell dysfunction, as a result of impaired proliferative potential and/or increased apoptosis, is thought to be one of the causes contributing to the decreased muscle regeneration capacity in atrophy. We have previously shown that electrical stimulation improved satellite cell dysfunction. Here we test whether electrical stimulation can also enhance satellite cell proliferative potential as well as suppress apoptotic cell death in disuse-induced muscle atrophy. Eight-week-old male BALB/c mice were subjected to a 14-day hindlimb unloading procedure. During that period, one limb (HU-ES) received electrical stimulation (frequency: 20 Hz; duration: 3 h, twice daily) while the contralateral limb served as control (HU). Immunohistochemistry and western blotting techniques were used to characterize specific proteins in cell proliferation and apoptosis. The HU-ES soleus muscles showed significant improvement in muscle mass, cross-sectional area, and peak tetanic force relative to the HU limb (p<0.05). The satellite cell proliferative activity as detected within the BrdU+/Pax7+ population was significantly higher (p<0.05). The apoptotic myonuclei (detected by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) and the apoptotic satellite cells (detected by cleaved Poly [ADP-ribose] polymerase co-labeled with Pax7) were reduced (p<0.05) in the HU-ES limb. Furthermore the apoptosis-inducing factor and cleaved caspase-3 were down-regulated while the anti-apoptotic Bcl-2 protein was up-regulated (p<0.05), in the HU-ES limb. These findings suggest that the electrical stimulation paradigm provides an effective stimulus to rescue the loss of myonuclei and satellite cells in disuse muscle atrophy, thus maintaining a viable satellite cell pool for subsequent muscle regeneration. Optimization of stimulation parameters may enhance the outcome of the intervention.